Methods and Systems for Controlling the Products of Combustion

ABSTRACT

The present invention relates to methods and systems for controlling a combustion reaction and the products thereof. One embodiment includes a combustion control system having an oxygenation stream substantially comprising oxygen and CO 2  and having an oxygen to CO 2  ratio, then mixing the oxygenation stream with a combustion fuel stream and combusting in a combustor to generate a combustion products stream having a temperature and a composition detected by a temperature sensor and an oxygen analyzer, respectively, the data from which are used to control the flow and composition of the oxygenation and combustion fuel streams. The system may also include a gas turbine with an expander and having a load and a load controller in a feedback arrangement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/062,442 filed Mar. 4, 2011 entitled METHODS AND SYSTEMS FORCONTROLLING THE PRODUCTS OF COMBUSTION, which is the National Phaseapplication of PCT/US2009/055544 filed Aug. 31, 2009 entitled METHODSAND SYSTEMS FOR CONTROLLING THE PRODUCTS OF COMBUSTION, claims thebenefit of U.S. Provisional Application No. 61/105,331 filed Oct. 14,2008.

FIELD OF THE INVENTION

Embodiments of the invention relate to methods and systems forcontrolling the products of combustion. More particularly, methods andsystems for obtaining substantially stoichiometric combustion in anoxy-fuel type combustion reaction are provided.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, whichmay be associated with exemplary embodiments of the present disclosure.This discussion is believed to assist in providing a framework tofacilitate a better understanding of particular aspects of the presentinvention. Accordingly, it should be understood that this section shouldbe read in this light, and not necessarily as admissions of prior art.

Some approaches to lower CO₂ emissions include fuel de-carbonization orpost-combustion capture. However, both of these solutions are expensiveand reduce power generation efficiency, resulting in lower powerproduction, increased fuel demand and increased cost of electricity tomeet domestic power demand. Another approach is an oxy-fuel gas turbinein a combined cycle. However, there are no commercially available gasturbines that can operate in such a cycle.

The original oxy-fuel concept is based on the combustion of hydrocarbonswith pure oxygen, which results in extremely high temperatures. Suchhigh temperatures lead to problems in combustor life and also thedevelopment of Polycyclic Aromatic Hydrocarbons (PAHs), which lead tosoot production. Numerous solutions to these issues have been attemptedwith varying levels of success, including using carbon dioxide as a massflow gas through the turbine instead of air. However, this approach andothers require specialized turbine equipment that is not yetcommercially available.

U.S. Pat. No. 5,724,805 discloses a power plant having a gas turbinefueled by hydrocarbons mixed with an oxygen/carbon dioxide mixture.However, the disclosure states that the O₂/CO₂ mixture has more O₂ thanair and therefore burns at very high temperatures requiring a largecombustor chamber to allow time for the combustion gasses to graduallycool so less carbon monoxide (CO) is produced. As such, a specialized,non-standard combustor must be used for the techniques in the '805reference to be practiced.

As such, there is still a substantial need for methods and systems toeffectively control the temperature and composition of combustionproduct streams.

SUMMARY OF THE INVENTION

One embodiment of the present invention discloses a combustion controlsystem. The combustion control system comprises a combustor consistingof at least a primary combustion zone and a burnout zone; a highconcentration carbon dioxide (CO₂) supply (the diluent supply); anoxygen supply stream configured to combine with at least a first portion(the primary diluent flow) of the high concentration CO₂ stream to forman oxygenation stream substantially comprising oxygen and CO₂ and havingan oxygen to CO₂ ratio; and a combustion fuel stream with a flow and acomposition. The system further includes a combustor configured to mixand combust the oxygenation and combustion fuel streams within a primarycombustion zone and a burnout zone in which a second part of the diluentsupply (the secondary diluent) is added to form combustion productsstream with a temperature and a composition; at least one temperaturesensor configured to measure the temperature of the combustion productsstream after the exit of the combustor, wherein the temperature of thecombustion products stream is used to adjust the flow rate of thesecondary diluent to obtain the desired temperature at the exist of thecombustor; and at least one oxygen analyzer configured to measure theamount of oxygen in the composition of the combustion products stream,wherein the amount of oxygen in the combustion product is used toregulate the flow rate of the oxygen supply stream to achievesubstantially stoichiometric combustion.

In some embodiments, the combustion fuel stream may be comprised of atleast a high quality fuel gas stream, a low heating value fuel gasstream, and optionally, a high concentration CO₂ makeup stream. Theseparate streams may be operatively connected to a summation controllerconnected to the flow controllers for the individual streams to controlthe flow and composition of the combustion fuel stream to regulate thetemperature of combustion and avoid flame burnout. In some embodiments,each of the streams may be operatively connected to a flow controllercontrolled by a central control system.

In additional embodiments, the combustor may include a first mix zoneconfigured to mix the first portion of the high concentration CO₂ streamand the oxygen supply stream to form the oxygenation stream; a primarycombustion zone configured to house the combustion reaction whichproduces the combustion products stream; and a burnout zone configuredto deliver the second portion of the high concentration CO₂ stream tothe combustor to regulate the temperature of the combustor and thecombustion products stream. In one exemplary embodiment, a catalyst isadded to the initial high temperature combustion zone to catalyze thecombustion reaction. In another alternative embodiment, the second mixzone may be configured to pre-mix the oxygenation and combustion fuelstreams or concurrently mix the streams with the combustion reaction.

Another embodiment of the present invention discloses a combustioncontrol method. The method comprising providing a high concentration CO₂stream, an oxygen supply stream, and a combustion fuel stream, whereineach stream has a flow rate and a composition; combining at least afirst portion of the high concentration CO₂ stream and oxygen supplystream to form an oxygenation stream; combusting the oxygenation streamand the combustion fuel stream in a combustor to form a combustionproducts stream with a temperature and a composition; sensing the oxygencontent of the combustion products stream; and adjusting the flow rateof the oxygen supply stream until the combustion products stream issubstantially stoichiometric.

In a third embodiment of the present invention, a combustion system isprovided. The combustion system includes a combustion fuel streamcomprising substantially hydrocarbons and carbon dioxide (CO₂) andhaving an initial fuel to CO₂ ratio; an oxygenation stream comprisingsubstantially oxygen and carbon dioxide (CO₂), wherein the combustionfuel stream and the oxygenation stream are combined to form thecombustion reactants stream having a combined fuel to oxygen ratioregulated to meet a desired equivalence ratio (defined as the ratio ofthe actual fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizerratio) and a combined initial CO₂ to fuel ratio regulated to provide adesired combustion temperature within the primary combustion zone; asecondary diluent comprising substantially carbon dioxide (CO₂); and acombustor configured to combust the combustor inlet stream to producethe primary combustion product comprising substantially water and carbondioxide, wherein the primary combustion product is mixed with thesecondary diluent to form a combustion products stream having atemperature and a final CO₂ to fuel ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the present invention may becomeapparent upon reviewing the following detailed description and drawingsof non-limiting examples of embodiments in which:

FIGS. 1A-1E illustrate four alternative exemplary control schematics inaccordance with certain aspects of the present invention;

FIG. 2 illustrates a schematic of an exemplary combustor as it might beconfigured for use in the alternative exemplary systems of FIGS. 1A-1E.

FIG. 3 is an exemplary flow chart of a method of operating the system ofFIGS. 1A-1E;

DETAILED DESCRIPTION

In the following detailed description section, the specific embodimentsof the present invention are described in connection with preferredembodiments. However, to the extent that the following description isspecific to a particular embodiment or a particular use of the presentinvention, this is intended to be for exemplary purposes only and simplyprovides a description of the exemplary embodiments. Accordingly, theinvention is not limited to the specific embodiments described below,but rather, it includes all alternatives, modifications, and equivalentsfalling within the true spirit and scope of the appended claims.

The term “stoichiometric combustion,” as used herein, refers to acombustion reaction having a volume of hydrocarbons (e.g. fuel) and avolume of oxygen, where the volume of oxygen is just enough to combustor burn all or nearly all of the volume of hydrocarbons to produce avolume of combustion products having almost no oxygen remaining andalmost no hydrocarbons remaining.

The term “primary residence time,” as used herein, is the time requiredin a combustor to produce a combustion products stream at aboutequilibrium conditions at the local conditions of pressure andtemperature.

Embodiments of the present disclosure provide combustion processes andsystems designed for oxy-fuel combustion in a gas turbine. Preferredembodiments of the invention address problems associated with hightemperature oxy-fuel combustion, such as the development of PolycyclicAromatic Hydrocarbons (PAH's), which lead to soot production andproduction of problematic combustion products such as oxygen and carbonmonoxide (or similar products of incomplete combustion). One exemplaryembodiment of the combustion system design includes a high concentrationcarbon dioxide (CO₂) stream that is divided into at least a primarydiluent and secondary diluent and an oxygen supply stream configured tocombine with the primary diluent to produce an oxygenation streamsubstantially comprising oxygen (O₂) and carbon dioxide (CO₂) (e.g.“synthetic air”). The system further includes a combustion fuel streamand a combustor, wherein the combustor, which consists of at least aprimary combustion zone and a burnout zone, is configured to mix andcombust the combustion fuel stream and the oxygenation streams in asubstantially stoichiometric combustion reaction to form a primarycombustion products stream substantially comprising water (steam) andCO₂. In addition, the primary combustion product stream may be dilutedwith the secondary diluent to form a secondary combustion productstream.

Note, that in some embodiments, a high pressure combustion (e.g. greaterthan about 10 atmospheres) process may be utilized. The adiabatic flametemperature in the primary combustion zone can be controlled by varyingthe ratio of CO₂ mixed with the oxygen when forming the oxygenationstream. The temperature of the combustion products stream may becontrolled independently to obtain the desired temperature or otherproperties of the combustion products at the exit of the combustor. Assuch, in some embodiments, the system will include a temperature sensorfor measuring the combustion products stream and the percentage amountof CO₂ in the combustion products stream may be increased to decreasethe temperature of the combustion products stream or decreased toincrease the temperature.

In some embodiments of the present invention, CO₂ and oxygen are mixedto make a “synthetic air” (e.g. an oxygenation stream). The amount ofCO₂ mixed with the oxygen provides a way to control the temperature ofthe primary combustion product stream and also another variable to helpcontrol the composition of the products of the combustion. The combustordesign may include quench ports to provide additional CO₂ to the burnoutzone to prevent the high temperatures of combustion from impinging onthe combustor can. Additional embodiments of the system include acontrol system that measures the amount of hydrocarbon going to thecombustor and calculates and controls the correct amount of oxygenneeded for the combustion. The control system will also utilize feedbackfrom instrumentation on the products of combustion to update the oxygensupply stream flow controller to ensure the desired combustion isachieved to provide the correct amount of oxygen to the oxygenationstream. A catalytic post combustion step is also optionally provided,which may be required depending on the hydrocarbon mixture that is usedfor the combustor. This catalytic step will reduce the oxygen in theprimary combustion products stream down to the low levels required toavoid serious corrosion problems in the enhanced oil recovery (EOR)facilities.

Some embodiments of the present invention include methods of operatingan oxy-fuel combustion system to provide the benefits and advantages ofthe disclosed systems. For example, one embodiment includes combiningthe combustion fuel stream with the oxygenation stream and combustingthese streams in a combustor to form the combustion products stream. Thecombustion fuel and oxygenation streams may be pre-mixed or concurrentlycombined and combusted and may include a catalyst in some embodiments,depending on the composition and rates of the various streams. Themethod further includes sensing or detecting the temperature and/orcomposition of the combustion product stream and adjusting the flow rateof at least one of the combustion fuel and oxygenation streams until thecombustion is at substantially stoichiometric conditions.

The methods and systems of the present disclosure may be utilized in avariety of applications, including a combustion gas turbine burnersystem. The gas turbine may be an integrated turbine operating on asingle shaft, a multiple-shaft system, or a non-integrated system withan external burner, and may even be utilized with an independentcompressor and hot gas expander, depending on the temperatures, volumes,and other variables of the particular system. The methods and systemsmay be utilized to beneficially increase combustion efficiency (e.g.reduce the amount of unburned or partially combusted fuel and/or oxygen)and provide greater control of turbine inlet temperature across a rangeof load conditions.

At least one benefit of the disclosed systems and methods includes theflexibility to use a commercial gas turbine combustion can type systemwith an oxy-fuel/co-generation type of system, such as the ultra-lowemission power generation systems and processes disclosed inUS2011/0000221. By controlling the amount of CO₂ mixed with the oxygento form the oxygenation stream, the temperature and composition of theprimary combustion products stream can also be controlled. Applicationof the disclosed systems and methods may avoid the need to develop a newcombustor can for a gas turbine, permitting the use of “off-the-shelf”gas turbine combustor technology in such a system.

The combustor utilized could be similar to those used in thegasification process where oxygen and hydrocarbons react in a reducingatmosphere using steam to moderate the temperature. In the presentinvention, CO₂ would be used in place of the steam to moderate thetemperature. Using steam is expensive and would also result in theformation of additional hydrogen in the products of combustion which isnot desired in the present cycle. By mixing the CO₂ with the oxygen, itmay also be possible to use a more conventional diffusion type combustorsimilar to those used in existing gas turbines where CO₂ would be usedinstead of air to cool the combustion liners. Combustion at nearstoichiometric conditions is preferred to eliminate the cost of excessoxygen removal.

Embodiments of the present invention provide additional benefits. Thepresent systems and methods enable an operator or automated system tocontrol the temperature of the primary combustion zone separately fromthe temperature of the combustion product stream and control theproducts of combustion, such as limiting the amount of corrosive carbonmonoxide and oxygen in the combustion product, therefore enabling theuse of the combustion product in enhanced oil recovery (EOR) operations,which require the elimination of such corrosive components.Additionally, the disclosed systems and methods can adapt to changes inthe quality of available fuel gas. For example, if a low heating value(e.g. less than 40 percent (%) methane) fuel gas is provided, such asfrom a low quality gas reservoir or a reservoir after CO₂ breakthrough,the systems and methods can adjust the ratio of oxygen in theoxygenation stream and/or add or increase the amount of high qualityfuel gas or spiking fuel gas (e.g. hydrogen gas) to the combustionprocess to maintain the proper temperature and composition in thecombustion products stream.

Referring now to the figures, FIGS. 1A-1D illustrate four alternativeexemplary control schematics in accordance with certain aspects of thepresent invention. In particular, FIG. 1A is a basic exemplary system.The system 100 includes a high concentration carbon dioxide (CO₂) stream102 that may be split into at least a primary diluent stream 102 a and asecond diluent stream 102 b, and an oxygen supply stream 104, which maybe combined with the primary diluent stream 102 a to form an oxygenationstream 106 having an oxygen to CO₂ ratio. A combustion fuel stream 108is also provided, which may be comprised substantially of methane (CH₄)or may include a mixture of light hydrocarbons, heavier hydrocarbons,hydrogen (H₂), and inert gasses, such as carbon dioxide, depending onthe source. A combustor (e.g. combustor can) 110 is also provided, whichin the preferred embodiment is divided into two parts, a primarycombustion zone 110 a and a burnout zone 110 b, and which is configuredto receive at least the oxygenation stream 106 and the combustion fuelstream 108, mix and combust the oxygenation and combustion fuel streams106 and 108 in the primary combustion zone 110 a at a desired flametemperature and primary residence time inside the combustor sufficientto produce a hot products stream (not shown) near equilibrium conditionsand then dilute the hot products stream with the secondary diluentwithin the burnout zone 110 b to form the combustion products stream 112a, which may be fed into an expansion device 111 (e.g. a gas turbine orhot gas expander), which is operatively connected to a load controller111′ to form an expanded products stream 112 b. The expanded productsstream 112 b may be split to form stream 113, which may form at least aportion of the high concentration CO₂ stream 102 and a secondary stream128, which may be utilized for enhanced oil recovery (EOR),sequestration, or another purpose. The system 100 further includes atleast one of a temperature sensor 114 and an oxygen analyzer 126configured to measure the temperature and oxygen content, respectively,of the combustion products stream 112 a or the expanded products stream112 b. Temperature data from the temperature sensor 114 is used tocontrol the flow rate of the secondary diluent stream 102 b and toregulate the temperature of the combustion products stream 112 a. Theflow rate of the oxygen supply 104 is adjusted in proportion to the flowrate of the combustion fuel supply 108. Oxygen data from the oxygenanalyzer 126 is used to adjust the proportioning factor of the flow rateof the oxygen supply stream 104 to the combustion fuel supply 108 untila substantially stoichiometric combustion is achieved.

Still referring to FIG. 1, the system 100 further includes a centralcontroller 115 operatively connected to a first flow controller 116 afor controlling the primary diluent 102 a; a second flow controller 118for controlling the oxygen supply 104; a third flow controller 120 forcontrolling the combustion fuel stream 108; and a fourth flow controller116 b for controlling the secondary diluent stream 102 b. The centralcontroller 115 may also be connected to the temperature sensor 114 andthe oxygen sensor 126 to determine the amount of unburned oxygen in thecombustion products stream 112 a or the expanded products stream 112 band use those measurements to control the flow of the oxygen supplystream 104. The central controller 115 may also control the flow rate ofthe combustion fuel stream 108 and the oxygen supply stream 104 tomaintain desired stoichiometry as the load condition changes.

The high concentration carbon dioxide (CO₂) stream (or “diluent supplystream”) 102 may come from any convenient source. For example, at leasta portion of the diluent supply stream 102 may be derived from divertingor splitting at least a portion of the expanded products stream 112 bvia recycle stream 113. However, the system 100 may be located nearanother source of high concentration CO₂, such as an external pipelinenetwork, a high CO₂ gas well, a gas treatment plant, or other source. Inaddition, recycle stream 113 may include some treatment, such as afiltering system like a membrane, mole sieve, absorption, adsorption, orother system to remove potentially dangerous or undesirable components,such as un-reacted oxygen or hydrocarbons. In particular, if the oxygenanalyzer determines that the expanded products stream 112 b has highlevels of oxygen, then the expanded products stream 112 b should not beused as a diluent, like in the secondary diluent stream 102 b.Similarly, high levels of unreacted hydrocarbons may also beunacceptable, depending on the combustor 110 and may need to be removedor separated before use as a secondary diluent stream 102 b. However, itis preferred and intended that the combustion product stream 112 a hasundergone a substantially stoichiometric combustion, so it should haveless than about 3.0 volume percent (vol %) oxygen, or less than about1.0 vol % oxygen, or less than about 0.1 vol % oxygen, or even less thanabout 0.001 vol % oxygen and less than about 3.0 volume percent (vol %)hydrocarbons, or less than about 1.0 vol % hydrocarbons, or less thanabout 0.1 vol % hydrocarbons, or even less than about 0.001 vol %hydrocarbons.

The secondary stream (or “remainder stream”) 128 may be used for sales,used in another process requiring high concentration carbon dioxide, orcompressed and injected into a terrestrial reservoir for enhanced oilrecovery (EOR), sequestration, or another purpose. Like with recyclestream 113, stream 128 may need to undergo some conditioning before useto remove potential contaminants or reactants like nitrogen oxides (NOx)or oxygen. Again, it is preferred and intended that stream 104 includessubstantially no nitrogen, and that stream 112 a has undergone asubstantially stoichiometric combustion, so it should have less thanabout 3.0 volume percent (vol %) oxygen, or less than about 1.0 vol %oxygen, or less than about 0.1 vol % oxygen, or even less than about0.001 vol % oxygen and less than about 3.0 volume percent (vol %) NOx,or less than about 1.0 vol % NOx, or less than about 0.1 vol % NOx, oreven less than about 0.001 vol % NOx.

The oxygen supply stream 104 may be provided by an air separation unit(ASU) or other process or system providing high purity oxygen. Theseparated nitrogen may be used in another related process, such as in anitrogen injection well as disclosed in US2011/0000221. In one exemplaryembodiment, the oxygen supply stream 104 may include from about 90 vol %to about 99.9 vol % oxygen with the remainder argon and may includetrace amounts of nitrogen and carbon dioxide. In another exemplaryembodiment, the oxygen supply stream may include from about 95 vol % toabout 96 vol % oxygen with about 4 vol % to about 5 vol % argon and lessthan about 0.2 vol % carbon dioxide.

The central controller 115 may be any type of control system configuredto receive data inputs, such as flow rates and compositions, and sendsignals to control flow rates via, for example, valves, pumps,compressors, and any other device that may be used to adjust a flowrate. In one exemplary embodiment, the central controller 115 mayinclude a programmable computer having user input devices such as akeyboard and mouse, user output devices such as a monitor and speakers,and may operate using active memory (RAM), and be operably connected tohard disk drives, optical drives, network drives, and databases via aLAN, WAN, Wi-Fi, or other external network.

The flow controllers 116 a, 116 b, 118, and 120 may include programmableautomated controllers for receiving and processing signals from thecentral controller 115, and may be operably connected to flow valves orvanes, vents, or other means of increasing or decreasing the flow of asubstantially gaseous stream. Additionally, in one exemplary embodiment,the flow controllers 116 a, 116 b, 118, and 120 may be operablyconnected to flow and/or composition sensors, which may provideadditional data input, such as to verify changes in the flow rates ofthe respective streams controlled by the flow controllers. In order tomaintain flame stability and effective control, it may be beneficial toutilize a high speed controller for any or all of the controllers 116 a,116 b, 118, and 120.

Although flow controller 116 b may be an active sensor as describedabove, the flow rate of the secondary diluent stream 102 b may beuncontrolled in one exemplary embodiment. For example, the combustor 110may include a liner having one or more quench ports (e.g. dilutionholes) with a particular pattern and hold size designed to providedilution and control temperatures in the combustor 110. Hence, the flowrate of the secondary diluent stream 102 b may be primarily dependentupon the hardware design of the quench ports in the combustor 110 andthe pressure, temperature and composition of diluent supply stream 102.Additionally, the flow controller 116 b may still be useful for shuttingoff the flow of secondary diluent 102 b in case of shut down,contamination of the secondary diluent 102 b, or some other reason. Insome embodiments, the central controller 115 may further include two outof three voting for certain sensors, such as the temperature sensor 114and the oxygen analyzer 126. The control system, including the centralcontroller 115 may also be configured with at least one safety interlockand/or shutdown logic and an alarm if the system 100 gets out of controlto protect the downstream machinery.

The temperature sensor 114 may be a single sensor or may additionallyinclude a backup sensor for redundancy or an array of sensors in andaround the combustion products stream 112 a or the expanded productsstream 112 b to ensure accurate temperature readings. Any type ofappropriate temperature sensor may be used, although the sensor chosenshould have a high resistance to heat and be able to effectively operateat temperatures at or above about 2,000 degrees Fahrenheit (° F.) oreven above about 2,200° F. In some exemplary embodiments of thedescribed inventive system 100, the temperature sensor 114 may send datadirectly to the CO₂ flow controller 116 b, or may send data to thecentral controller 115, which then controls the response of the flowcontroller 120. Alternatively, the temperature sensor 114 may also senddata directly to the combustion fuel stream flow controller 120.Additionally and alternatively, the temperature sensor 114 may take datafrom inside the combustor 110 near the exhaust or downstream of theburnout zone 110 b after exiting, at multiple locations along thecombustion products stream 112 a and expanded products stream 112 b, orsome combination thereof. The temperature of the streams 112 a and 112 bshould be limited to within certain operating parameters, which willdepend highly on the equipment in use, the type of combustion fuelstream and other input streams available, the potential uses for theremainder stream 128, and other factors.

Generally, the temperature in the primary combustion zone 110 a shouldbe below about 3,500° F. to avoid NOx production and because mostcommercial combustors 110 cannot operate above such temperatures, butthis limitation may be set higher if the material of the combustor 110can operate at higher temperatures and there is no nitrogen in thesystem 100. The temperature is preferably less than about 2,500° F. atthe inlet of the expander 111. Such high temperatures also contribute tothe formation of undesirable Polycyclic Aromatic Hydrocarbons (PAH's),which lead to soot production. However, the temperature in the primarycombustion zone 110 a must be sufficiently high to avoid flame burnout,which is done by regulating the oxygen to CO₂ ratio based on thetemperature of the reactants entering the primary combustion zone andthe heat release available from the specific fuel 108 and sufficientlyhigh to effectively combust substantially all of the oxygen (O₂) andhydrocarbons (e.g. stoichiometric combustion temperature) to produce theexpanded products stream 112 b requiring only limited conditioningbefore use in enhanced oil recovery (EOR) or as a diluent in the system100. For many cases, the preferred temperature of the combustion productstream 112 a will be from at least about 1,500° F. to at most about2,500° F. or from at least about 1,600° F. to at most about 1,900° F.For many cases, the preferred adiabatic flame temperature within theprimary combustion zone will be from at least 2,450° F. to at most3,500° F. unless improved materials of construction and no nitrogen ispresent in the combustion reactants in which case the upper limit may beincreased.

The oxygen analyzer 126 may be a single sensor, may include additionalsensors for redundancy, or an array of sensors at multiple locations toensure accurate measurements. For example, a plurality of lambda orwideband zirconia O₂ sensor may be used to provide feedback to one ofthe central controller 115 and/or the oxygen supply stream flowcontroller 118. If the lambda sensor is used, the central controller 115may be configured to dither the ratio of the fuel in the combustion fuelstream 108 to the oxygen in the oxygen supply stream 104 as the oxygencontent of the combustion products stream 112 a varies from astoichiometric coefficient below 1.0 to above 1.0. The dithering processis similar to those used in the automotive industry for internalcombustion engines. In any case, the oxygen content of the combustionproducts stream is preferably low, from less than about 3.0 volumepercent (vol %) to less than about 1.0 vol % to less than about 0.1 vol% to less than about 0.001 vol %. If the amount of oxygen is too high,then the flow rate of the oxygen supply stream 104 is reduced. In turn,this may lower the flame temperature, as discussed above, requiring anadjustment of the flow of the combustion fuel stream 108.

FIG. 1B illustrates the basic exemplary system as shown in FIG. 1A, butwith additional, optional features configured to further treat orcondition the products streams 112 a and 112 b. As such, FIG. 1B may bebest understood with reference to FIG. 1A. The system 140 includes allof the features disclosed with respect to FIG. 1A, but further includesa post-combustion catalysis apparatus 146 configured to reduce theoxygen and carbon monoxide content in the products streams 112 a and 112b and a combustion fuel bypass stream 142 with a flow and a compositionand having a flow controller 144 for controlling the flow rate of thecombustion fuel bypass stream 142. The oxygen analyzer 126 may beoperatively connected to the flow controller 144 directly or indirectlyvia the central controller 115. Additional flow controllers and oxygenanalyzers (not shown) may be required in certain specific embodimentswhere the combustion fuel bypass stream 142 is split or stream 128 islooped, as discussed further below.

The catalysis apparatus 146 may be a single device or a plurality ofdevices in parallel or series, but is preferably a small devicerequiring only a small amount of power to operate. In particular, thecatalysis apparatus 146 may be a carbon monoxide reduction catalystand/or an oxygen reduction catalyst that is normally used in HeatRecovery Steam Generators (HRSG's) to meet emissions requirements. Sucha system is generally not designed to remove large amounts of oxygen,but if significant amounts of oxygen remain in the expanded productsstream 112 b, the expanded product stream 112 b may need to recyclethrough the catalysis apparatus 146 more than once before it iscompressed and injected for enhanced oil recovery (EOR). As such, insome embodiments, another oxygen analyzer (not shown) may be neededafter the catalysis apparatus 146 to ensure that the injection stream128 is sufficiently low in oxygen (e.g. less than about 0.5 volumepercent (vol %) oxygen or less than about 0.1 vol %) to avoid corrosionof the compression and injection equipment and avoid souring thereservoir by injecting oxygen that can react with the hydrocarbonsremaining in the reservoir.

The combustion fuel bypass stream (e.g. second portion of the combustionfuel stream) 142 is configured to be combined with the expanded productsstream 112 b downstream from where recycle flow stream 113 is dividedfrom the expanded product stream 112 b, and is preferably introducedupstream from the catalysis apparatus 146 so that the additionalhydrocarbons may be used in the catalysis apparatus 146 to improveoxygen removal efficiency. However, in some alternative embodiments, thebypass stream 142 may be split and introduced before and after thecatalysis apparatus 146. In the embodiment where the EOR stream 128 islooped back to the catalysis apparatus 146, it may be beneficial tointroduce a portion of the bypass stream 142 into the EOR stream 128before looping it back to the catalysis apparatus 146. Beneficially, thebypass stream 142 is configured to reduce the volume percent of oxygenin the EOR stream 128 before compression and injection into a reservoirto substantially avoid corrosion of injection and compression equipmentand souring the hydrocarbons remaining in the injection reservoir.

FIG. 1C is an illustration of a third exemplary embodiment of the systemof FIG. 1A, which may or may not include the features disclosed in theillustration of the embodiment of FIG. 1B. As such, FIG. 1C may be bestunderstood with reference to FIGS. 1A and 1B. The system 150 includes ahydrocarbon analyzer 152 configured to measure the amount ofhydrocarbons in the composition of the products streams 112 a and/or 112b, a high quality fuel gas supply 108 a controlled by a flow controller154, and a low heating value fuel gas supply 108 b controlled by a flowcontroller 156. Flow controller 156 may be directly connected to thehydrocarbon analyzer 152 and/or may be connected via central controller115. The flow controllers 154, 156, and optionally 120 may beoperatively connected to a summation controller 158, which may beconnected to the central controller 115 directly or via oxygen supplycontroller 118.

The high quality fuel gas stream 108 a may be comprised of substantiallypure methane (e.g. about 99 vol %) and alternatively may comprise a“spiking” fuel gas such as hydrogen, higher hydrocarbons (C₃+) or anycombination thereof. The composition of the high quality fuel gas stream108 a will primarily vary depending on the needs of the system 150 andthe availability of various fuel types, but preferably will not includesignificant quantities of inert gases (e.g. nitrogen, carbon dioxide,etc.) or acid gases (e.g. sulfur dioxide, hydrogen sulfide, etc.). Highquality fuel gas stream 108 a may be from any reasonable source, but ispreferably available from a nearby gas production field rather thanimported from a significant distance. Specifically, if the stream 108 ais hydrogen, it may be provided from an auto-thermal reforming (ATR)process performed on a gas production stream from a nearby productiongas field (not shown).

The low heating value fuel gas stream 108 b may be comprised of lessthan about 80 vol % methane, less than about 60 vol % methane, less thanabout 40 vol % methane, or even less than about 20 vol % methane. Thelow heating value stream 108 b may also include small amounts of heavierhydrocarbons such as ethane and propane. In most cases, the majority ofthe remainder of the stream 108 b will be inert gases such as carbondioxide, but in some cases, there will be small amounts of nitrogen,hydrogen sulfide, helium, and other gases. Preferably, allnon-hydrocarbons and all inert gases other than carbon dioxide will beseparated out of the stream 108 b prior to mixing and combustion.

In one exemplary embodiment, the flow and composition of the twohydrocarbon containing streams 108 a and 108 b are used to calculate theoxygen requirement to operate the combustor 110 and provide the setpoint for the oxygen supply flow controller 118. The calculation willprovide the amount of oxygen needed for a stoichiometric combustion inthe combustor 110. The flows and compositions of the streams may changeover the life of the system 150, depending on the source of the streams108 a and 108 b. For example, the low heating value fuel gas 108 b mayoriginate from an EOR well having a high methane content in earlyproduction (e.g. above about 80 vol %). In such a case, there may belittle or no flow through the high quality fuel gas stream 108 a.However, when breakthrough occurs, the flow from the low heating valuefuel gas stream 108 b may comprise very low methane content (e.g. lessthan about 20 vol %). In that case, the flow from the high quality fuelgas stream 108 a is increased to add hydrocarbons to the combustion fuelstream 108.

FIG. 1D is an illustration of a fourth exemplary embodiment of thesystem of FIG. 1A, which may or may not include the features disclosedin the illustration of the embodiment of FIGS. 1B and 1C. As such, FIG.1D may be best understood with reference to FIGS. 1A-1C. The system 160further includes a high concentration CO₂ makeup stream 108 c with aflow and composition and a flow controller 162 operatively attachedthereto.

The CO₂ makeup supply stream 108 c may be combined with streams 108 aand 108 b to generate a combustion fuel gas stream 108 having asubstantially constant composition over the life of the system 160. Theapproach is similar to the system 150, but the physical characteristicsof the combustor could be designed specifically for the composition of108 and still burn fuels that have variable composition 108 b. The CO₂stream 108 c may be split from the expanded products stream 112 b ororiginate from another source.

FIG. 1E is an illustration of a fourth exemplary embodiment of thesystem of FIGS. 1A-1D. As such, FIG. 1E may be best understood withreference to FIGS. 1A-1D. The system 170 includes a combustion fuelstream 108 comprising substantially hydrocarbons and carbon dioxide(CO₂) and having an initial fuel to CO₂ ratio; an oxygenation stream 106comprising substantially oxygen and carbon dioxide (CO₂), wherein thecombustion fuel stream 108 and the oxygenation stream 106 are combinedto form a combustor inlet stream 172 having a combined fuel to oxygenratio configured to meet an optimal equivalence ratio and a combinedinitial CO₂ to fuel ratio configured to provide an optimal combustiontemperature; a secondary diluent stream 102 b; and a combustor 110configured to combust the combustor inlet stream 172 to produce a hotproducts stream 174 comprising substantially water and carbon dioxide,wherein the hot products stream 174 is mixed with the secondary diluentstream 102 b to form a combustion products stream 112 a having atemperature and a final CO₂ to fuel ratio.

In some exemplary embodiments, the hydrocarbons in the combustion fuelstream 108 are comprised substantially of methane and the fuel to oxygenmolar ratio is from about 1.9:1 mol fuel to mol oxygen to about 2.1:1mol fuel to mol oxygen or from about 1.95:1 mol fuel to mol oxygen toabout 2.05:1 mol fuel to mol oxygen. These molar ratios areapproximately equivalent to stoichiometric ratios of 0.9:1 to about1.1:1. In additional exemplary embodiments, the hydrocarbons in thecombustion fuel stream 108 are comprised substantially of methane andthe final CO₂ to fuel ratio is from about 10:1 mol CO₂ to mol fuel toabout 30:1 mol CO₂ to mol fuel or from about 15:1 mol CO₂ to mol fuel toabout 25:1 mol CO₂ to mol fuel or from about 20:1 mol CO₂ to mol fuel toabout 23:1 mol CO₂ to mol fuel.

In at least one exemplary embodiment, the system 170 further includes ahigh quality fuel gas stream 108 a with a flow and a composition; a lowheating value fuel gas stream 108 b with a flow and composition; and ahigh concentration CO₂ makeup stream 108 c configured to combine withthe high quality fuel gas stream 108 a and the low heating value fuelgas stream 108 b to form the combustion fuel stream 108 and maintain aconstant initial fuel to CO₂ ratio of the combustion fuel stream 108.Additional embodiments may include an oxygen supply stream 104 with aflow and a composition; and a high concentration CO₂ mixing stream 102 awith a flow and a composition configured to combine with the oxygensupply stream 104 to form the oxygenation stream 106.

In yet another exemplary embodiment, the system 170 additionallyincludes at least one temperature sensor 114 configured to measure thetemperature of the combustion products stream 112 a and/or expandedproducts stream 112 b (and optionally the hot products stream 174)wherein the temperature of the streams 112 a or 112 b are used tocalculate the flow rate of at least one of the high concentration CO₂mixing stream 102 a, the high concentration CO₂ makeup stream 108 c, andthe secondary diluent stream 102 b, to regulate the temperature ofcombustion; at least one oxygen analyzer 126 configured to measure theamount of oxygen in the composition of the products streams 112 a and/or112 b, wherein the amount of oxygen in the products streams 112 a-112 bis used to optimize the flow rate of the oxygen supply stream 104 toachieve substantially stoichiometric combustion; and at least onehydrocarbon analyzer 152 configured to measure the amount ofhydrocarbons in the composition of the products streams 112 a-112 b,wherein the amount of hydrocarbons in the composition of the productsstreams 112 a-112 b is used to optimize the flow rate of the oxygensupply stream 104 to achieve substantially stoichiometric combustion.The system 170 may also include a gas turbine 111 having a load and aload controller 111′ configured to measure the load, wherein the loadmeasurement is used to maintain the combined fuel to oxygen ratio as theload changes.

FIG. 2 illustrates a schematic of an exemplary combustor as it might beconfigured for use in the alternative exemplary systems of FIGS. 1A-1D.As such, FIG. 2 may be best understood with reference to FIGS. 1A-1D.The combustor system 200 comprises a first mix zone 202, a second mixzone 204, an initial high temperature primary combustion zone 205, aburnout zone 206, and optional fuel injection nozzles 208 a and 208 b.The hot products stream (or “primary combustion products stream”) 212 isproduced from primary combustion zone 205. Note, that in some exemplaryembodiments, a high pressure combustion process (e.g. greater than about10 atmospheres) may be utilized.

The first mix zone 202 is configured to mix the primary diluent stream102 a with the oxygen supply stream 104 to form the oxygenation stream106. The second mix zone 204 is configured to mix the oxygenation stream106 and the combustion fuel stream 108. In one embodiment the streams106 and 108 may be pre-mixed in the second mix zone 204 and thendirectly flow into the primary combustion zone 205. In anotherembodiment, which is referred to as non-premixed, the second mixing zone204 and the primary combustion zone 205 overlap and occur concurrently,as in a diffusion burner type of arrangement. The primary combustionzone 205 includes a burner, a flame, and the combustion reaction itself,which produces the hot products stream 212. The burnout zone 206delivers the secondary diluent stream 102 b to the combustor 110 toregulate the temperature of the combustor 110 and the combustionproducts stream 112 a.

In some exemplary embodiments of the combustor 110, the burnout zone 206comprises one of a passive dilution zone having a series of holesconfigured to cool and quench the liner of the combustor 110; an activedilution zone having at least one quench port configured to activelydeliver at least a portion of the secondary diluent stream 102 b to thecombustor 110 to mix with the primary combustion products stream 212; aseries of staged quench ports to actively control a temperature patternthrough the burnout zone 206; and any combination thereof. In addition,the burnout zone 206 may include a pressure transducer or other sensor210 to monitor and measure pressure oscillations in the combustor 110,which are a sign of flame blowout. An oxygen analyzer (not shown) mayalso be included in the combustor 110 to provide another input to theoxygen feedback loop.

In one exemplary embodiment of the non-premixed arrangement, thecombustion fuel streams 108 a and 108 b may be introduced in separatenozzles 208 a and 208 b that are configured based on the volume flow ofthe respective stream, or mixed and injected as a mixed combination ofthe combustion fuel stream 108 through a single injector.

The combination of the oxygenation stream 106 and the combustion fuelstream 108 in the combustor 110 is configured to maintain a minimumadiabatic flame temperature and flame stability to combust all or nearlyall of the oxygen in the oxygenation stream 106 (e.g. a stoichiometricreaction is preferred). In terms of heating value, the oxygenationstream 106 has no heating value, the high quality fuel gas stream 108 amay have a relatively high value (e.g. from at least 500 British thermalunits per standard cubic foot (BTU/scf) to about 950 BTU/scf) and thelow heating value fuel gas stream 108 b has a relatively low heatingvalue (e.g. from about 150 BTU/scf to about 450 BTU/scf).

The combustor 110 may be a standard external combustor or may be acustomized or modified combustor. Examples of applicable combustor typesinclude an oxyClaus burner, a partial oxidation (PDX) burner,auto-thermal reforming (ATR) burner, and gas turbine diffusioncombustors. Note that each burner type may require some modification towork with a substantially CO₂ stream.

In one exemplary embodiment, the second mix zone 204 and nozzles 208 aand 208 b may be configured to mix the combustion fuel stream 108 andthe oxygenation stream 106 in a highly turbulent manner to ensure ahomogeneous mixture is achieved. During operation, the primarycombustion zone 205 produces temperatures up to about 2,200° C. With theaddition of the quench gas 102 b, the combustion products stream 112 ais expected to be up to about 1,400° C. as it enters the burnout zone206. Additional quench gas 102 b may be introduced via the outer wall ofthe burnout zone 206 generating a sort of “gas envelope” to keep thewall of the combustor 110 notably cooler than the flame 205. In oneexemplary embodiment, the cooling stream 102 b may be stripped ofhydrocarbons to minimize soot formation, if necessary. In anotherexemplary embodiment, the combustion takes place at higher thanatmospheric pressure, such as above about 10 atmospheres. The reactiongenerates water (vapor) and carbon dioxide as shown by the equationsbelow (the carbon dioxide entering the chamber generally remainsunreacted):

CH₄+2O₂=2H₂O+CO₂

FIG. 3 is an exemplary flow chart of a method of operating the system ofFIGS. 1A-1D. As such, FIG. 3 may be best understood with reference toFIGS. 1A-1D. The method 300 begins at block 302, then includes providing304 a high concentration CO₂ stream 102, an oxygen supply stream 104,and a combustion fuel stream 108, wherein each stream has a flow rateand a composition; splitting 306 the high concentration CO₂ stream 102into a primary diluent stream 102 a and a secondary diluent stream 102b, each having a flow rate; adjusting 308 the primary diluent streamflow rate independently of the overall flow rate of the highconcentration CO₂ stream; combining 310 the primary diluent stream 102 aand the oxygen supply stream 104 to form an oxygenation stream (e.g.“synthetic air”) 106; combusting 312 the oxygenation stream 106 and thecombustion fuel stream 108 in a primary combustion zone 110 a of thecombustor 110 to form a combustion products stream 112 a with atemperature and a composition; sensing 314 the oxygen content of thecombustion products stream 112 a; and adjusting 316 the flow rate of theoxygen supply stream 104 until the combustion products stream 112 a issubstantially stoichiometric using the sensed oxygen content. In oneembodiment, the method 300 includes sensing 314 the temperature of thecombustion products stream 112 a and adjusting 316 the flow rate of theprimary diluent stream 102 a to regulate the temperature of combustionusing the sensed temperature. In one embodiment, the method includesdirecting 318 a portion of the combustion products stream 112 a to anenhanced oil recovery (EOR) process.

Examples

Some exemplary gas stream compositions are provided in the tables belowas examples of gas streams at different stages of production in a singlegas production field, or different gas production fields. Table 1provides specific stream compositions and flow rates for a productionwell at or near the beginning of production.

TABLE 1 Start-up Stream Stream Stream Stream Stream Component 104 108b108a 102a 112 O₂ 95.59% 0 0 0 0.44% CO₂ 0 0 0 76.23% 61.83%  CH₄ 0 0  100% 0 0.00% CO 0 0 0 0 0.85% OH 0 0 0 0 0.12% H₂ 0 0 0 0 0.09% H₂O 00 0 16.99% 30.42%  Ar  4.26% 0 0  6.78% 6.34% Misc  0.15% 0 0 0 0.00%Total 100.00%  0.00% 100.00% 100.00%  100.09%  Pressure psig 300 300 300300 250 Temp (° F.) 755 500 160 540 1701.7 LB Moles 13474.1 0 6464.1143859.5 163798 Flow (lb/hr) 436010 0 103425 6282874 6822309Table 2 provides specific stream compositions and flow rates for aproduction well after CO₂ breakthrough.

TABLE 2 Post Breakthrough Stream Stream Stream Stream Stream Component104 108b 108a 102a 112 O₂ 95.59% 0 0 0 0.014% CO₂ 0 88.16%  0 0 64.15%CH₄ 0 5.21%   100% 0  0.00% C₂ 0 2.76% 0 0  0.00% C₃ 0 1.25% 0 0  0.00%CO 0   0% 0 0 0.028% OH 0   0% 0 0 0.004% H₂ 0   0% 0 0 0.236% H₂O 0  0% 0 0 31.02% N₂ 0   1% 0 0  0.84% Ar  4.26% 0 0 0  0.40% Misc  0.15%1.77% 0 0  3.3% Total 100.00%  100.00%  100.00% 0.00% 100.09%  Pressurepsig 300 300 300 300 250 Temp (° F.) 755 500 160 540 1701.7 LB Moles13474.1 136739.7 171.8 0 150386 Flow (lb/hr) 412653 5639146 2748 06054547

While the present invention may be susceptible to various modificationsand alternative forms, the exemplary embodiments discussed above havebeen shown only by way of example. However, it should again beunderstood that the invention is not intended to be limited to theparticular embodiments disclosed herein. Indeed, the present inventionincludes all alternatives, modifications, and equivalents falling withinthe true spirit and scope of the appended claims.

1. A system, comprising: a combustor that includes a combustion fuelport, an oxygen supply port, a diluent supply port, and a combustionproducts output port; an oxygen analyzer that measures an amount ofoxygen in the combustion products stream after exiting the combustorthrough the combustion products output port; and a control system that:controls a flow rate of a combustion fuel stream entering the combustorthrough the combustion fuel port, controls a flow rate of an oxygensupply stream entering the combustor through the oxygen supply port, andadjusts a proportioning factor between the flow rate of the oxygensupply stream and the flow rate of the combustion fuel stream, whereinadjustment of the proportioning factor is based on the amount of oxygenin the combustion products stream determined by the oxygen analyzer. 2.The system of claim 1, further comprising a temperature sensor thatmeasures a temperature of a combustion products stream after exiting thecombustor through the combustion products output port.
 3. The system ofclaim 2, wherein the control system adjusts a flow rate of a diluentsupply stream into the diluent supply port, which causes the combustionproducts stream to achieve a predetermined temperature.
 4. The system ofclaim 1, wherein the control system controls the flow rate of the oxygensupply stream independent of an adjustment to a diluent supply stream.5. The system of claim 1, wherein the control system adjusts theproportioning factor until a substantially stoichiometric combustion,within the combustor, is achieved.
 6. The system of claim 1, wherein thecontrol system adjusts the proportioning factor by adjusting the flowrate of the oxygen supply stream.
 7. The system of claim 1, wherein asubstantially stoichiometric combustion results in the combustionproducts stream including less than about 3.0 volume percent (vol %)oxygen and less than about 3.0 volume percent (vol %) NOx.
 8. The systemof claim 1, further comprising: an expansion device that receives thecombustion products and expands the combustion products, wherein thecontrol system controls the flow rate of the combustion fuel stream tomaintain a predetermined load condition in the expansion device.
 9. Thesystem of claim 1, wherein the control system controls a desiredcombustor stoichiometry based on a condition of a load communicativelycoupled to the combustion products output port changes.
 10. The systemof claim 1, wherein the oxygen analyzer is a lambda oxygen sensor. 11.The system of claim 1, wherein the oxygen analyzer is a widebandzirconia oxygen sensor.
 12. The system of claim 1, wherein the controlsystem adjusts the proportioning factor by adjusting the flow rate ofthe oxygen supply stream.
 13. The system of claim 1, wherein the controlsystem includes a plurality of controllers.